Polyesters containing hydroxyl and/or carboxyl groups are known and may be produced by various methods. Thus, polyesters can be obtained, for example, by self-catalysis from dicarboxylic acids and diols. However, this process is highly laborious in the case of aromatic dicarboxylic acids of the type preferably used for the production of polyesters for powder lacquers, so that reaction times of longer than 50 h before a defined, low terminal group concentration is adjusted are by no means rare.
Free acids cannot be used as catalysts in cases such as these, because the acid interferes with the use of the end product as a binder; on the other hand, separation is not possible during working up. Instead, the methyl ester rather than the free acid and a transesterification catalyst are normally used and methanol is distilled off from the reaction mixture. However, the catalysts used for the transesterification often have to be inactivated during the reaction and replaced by condensation catalysts because otherwise the molecular weight built up is inadequate. The catalysts used for this purpose are generally soluble compounds of Zn, Cd, Mn, Ti, Sn and Sb.
Apart from the inadequate acceleration of the reaction, disadvantages of this process include the discoloration which often occurs and the fact that the catalyst can no longer be removed from the reaction product.